Ellipsis, susbstition, emphasis
He can speak French but seldom does he use / rarely he uses / not once does he use it in public.
He can speak French but seldom does he use it in public.
He uttered those words and at the same time all the newspapers published them. (No sooner)
No sooner had he uttered uttered those words than all the newspapers published them.
I realise only now the stress he was under. (Only now)
Only now do I realise the stress he was under.